The present invention relates to a mass flow controller for controlling a mass flow of a fluid such as a gas, and particularly a mass flow controller precisely controlling from a small amount of mass flow to a large amount of mass flow and an operating method thereof.
In the production of semiconductor devices, the film-forming process, etching treatment, etc. are carried out in a confined chamber by precisely controlling the mass flow of a small amount of a process gas. As the means for controlling the mass flow, a mass flow controller has been generally used.
FIG. 10 is a vertical sectional view showing a conventional mass flow controller 101. As seen from FIG. 10, the conventional mass flow controller 101 mainly comprises a sensor means 102 for detecting the mass flow of a fluid (hereinafter described on a gas as the example of the fluid), a mass flow control valve means 104 equipped with an actuator 140, and a control circuit 103 (details are not shown) for electrically controlling the mass flow control valve means 104 and the actuator 140. A gas introduced from a fluid inlet 111 passes through an inlet flow path 112, at which the flow path is branched to a by-pass flow path 116 and a sensor flow path 120. A major portion of the introduced gas passes through the by-pass flow path 116, and the amounts of gas passing through the by-pass flow path 116 and the sensor flow path 120 are regulated in a predetermined ratio. The gases passing through the respective branched flow paths 116 and 120 are combined again at an intermediate flow path 113. The combined gas flows into a fluid outlet 114 by passing through a valve seat 143 and an outlet flow path 115.
The sensor flow path is a U-shaped stainless tube of an internal diameter about 0.5 mm, and the upstream end thereof is in fluid communication with the inlet flow path 112 and the downstream end with the intermediate flow path 113. The sensor flow path 120 has an upstream heat-sensitive coil 121 and a downstream heat-sensitive coil 122 each wound around the sensor flow path 120, both the heat-sensitive coils constituting a bridge circuit (not shown) together with resistances. The sensor means 102 comprises these elements of the sensor flow path 120, the upstream and downstream heat-sensitive coils 121 and 122 and the bridge circuit. Since the heat-sensitive coils is maintained at a predetermined temperature higher than that of the gas flowing into the sensor flow path 120, the upstream heat-sensitive coil 121 loses heat when the gas passes through sensor flow path 120 to lower the temperature thereof. Then, the heated gas raises the temperature of the downstream heat-sensitive coil 122 to create a temperature difference between the heat-sensitive coils 121 and 122. Such a heat transfer is detected as an unbalanced voltage in the bridge circuit. The mass flow is thus measured because the detected potential difference is proportional to the mass flow.
The mass flow signal from the sensor means 102 is amplified by an amplifier circuit and input to the controlling circuit 103 where the mass flow signal is compared with a preset mass flow level and, if there is a difference between them, a driving signal (valve driving voltage) to minimize the difference is put to the actuator 140, thereby regulating the mass flow by the mass flow controlling valve means 104.
The degree of valve opening is controlled by the upward and downward motion of the actuator 140, which moves a metal diaphragm 142 toward or away from the sealing surface of a valve seat 143. Since the flow rate of the gas is usually small, the degree of valve opening is adjusted by the stroke of the upward and downward motion as small as about 10 .mu.m. Therefore, a piezo element stack has been used as the actuator 140 because a large thrust can be obtained by a small stroke of the motion. The mass flow control valve means 104 comprises the valve seat 143 disposed at the downstream end of the intermediate flow path 113, the metal diaphragm 142 for controlling the outlet opening of the valve seat 143, a valve rod 141 for pressing the diaphragm 142 interiorly including the actuator (piezo element stack) 140, and a spring means 146 for press contacting the valve rod 141 via the diaphragm 142 onto the sealing surface of the valve seat 143 thereby maintaining the valve closed before the control operation. The diaphragm 142 is a thin metal sheet having a flat central area and an annular spring portion having a semicircular cross section surrounding the flat central area. With the spring portion, the diaphragm 142 has a self-restoring force. The periphery of the diaphragm 142 is held by a press member. The upper end of the actuator 140 is fixed to a position adjusting member 145, and the lower end is abuttingly supported by a bridge member 144 which is fixedly mounted to a flow path body 110.
Upon applying a voltage to the actuator 140, the piezo element stack expands upward against the downward force from the spring means 146 because the lower end of the piezo element stack is supported on the bridge member 144. The upward expansion of the piezo element stack moves the valve rod 141 upward to reduce the pressing force of the lower end of the valve rod 141 applied on the diaphragm 142. As a result thereof, the diaphragm 142 tightly contacted to the sealing surface of the valve seat 143 is allowed to separate therefrom by its self-restoring force to increase the degree of the valve opening, thereby controlling the mass flow. The above description and FIG. 10 are made on a normally closed valve, and the fundamental functions are the same also in a normally opened valve.
The production process of semiconductor devices includes a step of completely shutting off the flow of the gas. Although the mass flow controlling valve is effective for controlling a small flow mass, not sufficient for completely shutting-off the fluid flow. Since the conventionally known mass flow controller does not completely shut off the fluid flow, it has been proposed, as shown in FIG. 11, to provide a mass flow controller M with a separate shut off valve assembly V operated manually or by air-cylinder in series at a downstream side thereof. In the proposed apparatus, an amount of gas gradually leaked from the mass flow control valve during the shut off mode is entrapped in a flow path F between the mass flow controller M and the shut off valve assembly V. The entrapped gas adversely affects the next mass flow control operation. For example, an excessively large amount of flow (overshoot flow) occurs immediately after the shut off valve is reopened in the next operation, and it takes a non-negligible time until the mass flow reaches the control point as shown in (b2) of FIG. 4(b). Thus, the entrapped gas deteriorates the response of the mass flow controller. To avoid the overshoot flow, the conventional process includes an additional step for expelling the entrapped gas from the flow path F just prior to the initiation of a next control. Also, when a different gas is used in the next control operation, an additional time for purging the flow path F is required.
In addition, the provision of the shut off valve assembly V increases the geometric size of the apparatus. The increase in the geometric size is unfavorable because an apparatus for producing semiconductor devices requires to put the pipeline arrangement including mass flow controller together in a cylinder cabinet.
Further, in the apparatus mentioned above, the mass flow controller 101 is controlled electrically by a control signal from the control circuit 103 and the shut off valve 105 is controlled pneumatically by an open-closes action of a solenoid valve. Such a control by different control systems makes a simultaneous control of the mass flow control and the open-close control of the shut off valve difficult. Therefore, a timing lag in the mass flow control and the open-close control of the shut off valve occurs to cause the overshoot flow.
The production process of semiconductor devices includes a step of supplying a process gas of a precise and small mass flow such as 1.+-.0.01 SCCM (cm.sup.3 /min at standard conditions) and a step of supplying an inert gas such as nitrogen of a great mass flow as large as about 20,000 SCCM to purge the flow paths of the mass flow controller. To conduct both the small mass flow step and the large mass flow step, an apparatus as shown in FIG. 12 has been proposed. As seen from FIG. 12, a mass flow controller 101 having a maximum controllable mass flow of about 1 SCCM has a bypass line 106 which is connected in parallel to the mass flow controller 101 so that the bypass line 106 bypasses the mass flow controller 101. The small mass flow control and the large mass flow purge are switched by opening or closing a shut off valve 105. However, the apparatus of FIG. 12 is large in its geometric size due to the auxiliary bypass line 106 and not suitable for use in the production of semiconductor device as mentioned above.
To eliminate the above problem, U.S. Pat. No. 5,421,365 proposes in FIG. 5 a flow control apparatus in which a bypass passage 48 is formed in the base block 40 for connecting a first passage 45 (intermediate flow path) and a second gas passage 46 (outlet flow path downstream the mass flow control valve), and a valve mechanism 49 (shut off valve) for opening and closing the bypass passage is provided in the inlet portion of the bypass passage. The bypass passage is used for passing a purging gas such as nitrogen to flow away the gas remaining in the pipe 23, and the first and second gas passages 45 and 46 when the gas flow adjusting mechanism 44 is troubled by a clogging, etc. When a trouble occurs in the gas flow adjusting mechanism 44, the valve mechanism 49 is opened so as to communicate the bypass passage 48 with the first and second gas passages 45 and 46. A part of the purge gas passing through the first gas passage 45 flows into the pipe 51 (sensor flow path) with small bore of the gas flow sensor 43.
Generally, the diameter of the bypass flow path is determined so as to ensure that a flow passing through the sensor flow path has a mass flow level sufficient for detecting when the mass flow is the maximum controllable mass flow. Therefore, the number of tubes of the bypass flow path is decreased with decreasing maximum controllable mass flow. For example, a mass flow controller having a small maximum controllable mass flow of about 1 SCCM has generally no bypass flow path, and all part of the gas passes through the sensor flow path. Since the sensor flow path has a small bore size and creates a large flow resistance, the mass flow controller cannot be purged by a sufficient amount of purge gas in a short period of time because of the large flow resistance. Therefore, in such a mass flow controller of a small maximum controllable mass flow, the flow paths cannot be purged sufficiently and a part of the process gas still remains in the flow paths of the mass flow controller or the pipe lines connected thereto. The remaining process gas is mixed with another process gas in the next production step and adversely affects the quality of semiconductor devices being produced.
Since the bypass passage 48 of U.S. Pat. No. 5,421,365 connects the first passage 45 (intermediate flow path) and the second gas passage 46 (outlet flow path), at least a part of the purge gas must flow through the sensor flow path having a small bore size to remarkably increase the flow resistance. This prevents the flow of a large amount of purge gas through the mass flow controller in a short time to result in an insufficient purge of the flow paths.
Further, the valve mechanism 49 of U.S. Pat. No. 5,421,365 merely controls on or off of the fluid flow passing through the bypass passage 48 when a trouble is happened in the main flow through the gas flow adjusting mechanism 44, and cannot avoid the problem of overshoot flow mentioned above because the valve mechanism 49 does not participate in shutting off the main flow of the gas from the fluid inlet to the fluid outlet through the gas flow adjusting mechanism 44.
Generally, the sealing surface of the valve seat is mirror-polished by lapping, and the valve seat thus finished is screwed or caulked to the flow path body. The sealing surface sometimes fails to being in parallel relation to the surface of the flow path body around thereof due to the inaccurate dimension of the valve seat and the uneven deflection of the valve seat from a proper fitting position. If not in parallel relation, a press means for holding the diaphragm and a bridge means are mounted inclined with respect to the sealing surface. Therefore, the uniform abutting of the metal diaphragm against the sealing surface cannot be obtained and the stroke of motion of the piezo element stack cannot be effectively transmitted to the metal diaphragm. Since the stroke is very small of micron order, the failure in transmitting the motion significantly affects the control of the mass flow utilizing the elastic deformation of diaphragm. Also, the inclined abutting of the diaphragm on the sealing surface of the valve seat deteriorates the accuracy of the mass flow control.